This invention pertains to the field of dynamic light scattering which is also known as quasi-elastic light scattering (QELS), intensity fluctuation spectroscopy (IFS), optical mixing spectroscopy, and photon correlation spectroscopy (PCS). More particularly, this invention pertains to the characterization of particle suspensions or dispersions using DLS. The invention is applicable to eye diagnostics such as the diagnosis of eye diseases (cholesterol deposit and blood sugar level in the anterior chamber, cataract in the lenses, diabetic retinopathy, age related molecular change in the vitreous, etc.) and will be described with reference thereto. However, it will be appreciated that the invention has broader applications such as monitoring the synthesis of microporous materials (e.g., zeolite), monitoring protein crystallization process, characterization of food proteins, analyzing skin and tissue, diagnosing biological fluids in situ and in vivo, on-line process monitoring, studying polymer induced aggregation and flocculation, studying highly concentrated and interacting systems, and characterizing diverse systems such as gels, solids, liquid crystals, colloidal suspensions, polyelectrolyte solutions, dispersions of microorganisms and solutions of viruses and biopolymers. Moreover, the invention may be advantageously employed in other environments and applications including static light scattering, Brillouin scattering, Raman scattering, fluorescence spectroscopy and laser doppler velocimetry and anemometry.
DLS is a widely used technique in studying the hydrodynamic properties of microscopic particles suspended in a fluid medium. Some of the information that can be obtained from DLS include the diffusion coefficient, average particle size, polydispersity, and particle size distribution.
The concept of DLS has been in use for the last half century, and numerous publications and books have been published on the subject. For example, the basic theory and experimental aspect of DLS can be reviewed in the book by Chu [Laser Light Scattering: Basic Principles and Practice, 1991].
In DLS, coherent light such as laser light illuminates microscopic particles dispersed or suspended in a fluid medium. The particles range in size from a few nanometers to a few microns. The particles scatter light over a wide range of angles by Rayleigh or Mie scattering. The intensity of the scattered light fluctuates in time due to the Brownian or thermal motion of the particles in the medium. The fluctuations of the light intensity contain information about the dynamics of the scattering particles. This information can be extracted by constructing a time correlation function (TCF). The TCF is computed using a digital correlator. In the case of dilute dispersions of spherical particles, the TCF provides quick and accurate determination of the translation diffusion coefficient of the particles. Using the Stokes-Einstein equation, the diffusion coefficient can easily be transformed into average particle size provided the viscosity of the suspending medium, its temperature, and refractive index are known.
A conventional apparatus for DLS experiments includes a source of laser light and a transmitting optical arrangement to launch the laser light into an illumination volume of a sample. The apparatus also includes a collecting optical arrangement to coherently collect the scattered laser light from a detection volume in the sample. The overlap of the illumination volume and the detection volume is the scattering volume.
A major problem in DLS systems is optical alignment. The optical arrangements must b e precisely aligned to obtain meaningful data. This is a tedious procedure that unfortunately limits the use of DLS to well-controlled environments such as a laboratory.
The recent use of fiber optics in DLS systems partially addresses the problem of optical alignment. First initiated by Tanaka and Benedek [Appl. Opt., 14, 189 (1975)], fiber optics provide several advantages over conventional optics in DLS systems. Fiber optics are compact, rugged, easy to handle, and portable.
The problem of optical alignment has also been partially re solved by integrating the transmitting and collecting optical arrangements into a single compact housing. This has allowed DLS to be used in areas such as biomedical diagnostics, remote sensing, and on-line process control. For example, Tahaka and Benedek showed that a compact fiber optic probe can be used to measure the velocity of the blood flow inside the human body. Other examples of fiber optic based DLS systems have been reviewed by Macfadyen [Optics and Laser Technology, 22, 175 (1990)].
In designing a DLS system, there are primarily two parameters that must be optimized: sensitivity and spatial coherence. Sensitivity is an indication of how high the scattered intensity would be with a given input laser power. Spatial coherence or signal to noise ratio is an indication of how much of the scattered light contains usable signal. Generally, optimizing one parameter can only be done at the expense of the other parameter. In other words, increased sensitivity can only be achieved by lowering spatial coherence, and vice versa. This relationship occurs because both parameters are controlled by the size of the scattering volume. The larger the scattering volume, the greater the intensity of collected light. However, the larger the scattering volume, the lower the spatial coherence. Therefore, a major design objective in DLS systems is an optimal balance between sensitivity and spatial coherence.
Some DLS systems use monomode optical fibers to transmit light through the system. Such systems can be separated into two groups--lensless probe designs and collimating probe designs. Lensless probe designs do not use lenses to transmit or receive laser light. Consequently, the beams from such systems are divergent. Collimating probe designs, on the other hand, use lenses to produce collimated beams of laser light in the scattering volume.
Lensless probe designs are exclusively used in the backscattering regime. Typically, they consist of a single fiber for transmission and reception of the laser light. Such designs suffer from a very small penetration depth (on the order of a few .mu.m) which is the distance between the probe, specifically the end face of the optical fiber, and the center of the scattering volume. This largely limits its application to the examination of immersible fluids wherein the probe is immersed in the fluid sample to obtain measurements.
Another problem with lensless designs is the heterodyning effect due to the internal reflection at the tip of the fiber. This necessitates a great deal of care in fabrication, and limits its application to scattering volumes containing high concentrations of suspended particles so that the scattered signal is strong enough to overcome the reflected signal.
In a two-fiber lensless probe, two monomode fibers are fixed to a housing to achieve a narrow angle between incident and back-scattered laser light. This increases the penetration depth although the depth is still only on the order of a few millimeters. The lensless probe suffers from low sensitivity and low spatial coherence. As a result, the probe is effective for only a limited range of particle sizes and concentrations. When using the probe with a low-power laser, such as in in vivo diagnostics, the range is still further limited. At greater penetration depths, the lensless probe has even lower sensitivity and spatial coherence.
In collimating probe designs, optical fibers are mated with lenses to produce collimated beams of laser light. In such designs, an end face of a monomode optical fiber is positioned at the Fourier plane (also called focal plane) of the lens to produce a collimated illumination volume. This approach, however, significantly compromises the sensitivity and spatial coherence of the probe.
There are essentially two types of optical fibers commercially available for use in DLS systems. One is multimode fiber and the other is monomode fiber or single-mode fiber. There is also polarization-maintaining fiber but it is generally considered to be a subclass of monomode fiber. All of the early fiber-optic DLS systems use multimode fibers because laser light couples more easily with a multi-mode fiber than with a monomode fiber. Further, until recently, no theoretical study showed any advantage of monomode fiber over multimode fiber in DLS systems. Indeed, multimode fiber collects much more scattered light than monomode fiber. However, multimode fiber has very poor spatial coherence. Thus, additional pinholes or apertures are needed in the optical arrangements to enhance the spatial coherence to an acceptable level.
The use of monomode optical fiber was first explored in Brown, J. Phys. E: Sci. Instrum., 20, 1312 (1987). Brown designed a 90.degree. DLS system using monomode optical fiber and micro lens. An advantage of monomode fiber is that it uniquely transmits only a single, spatially-coherent mode of light. This effectively eliminates the need for pinholes or apertures, as was pointed out by Ricka [Appl. Opt., 32, 2860 (1993)] in his theoretical study of monomode fiber.
Later, a multiangle DLS spectrometer system using monomode optical fiber and a graded index (GRIN) lens was taught in Dhadwal and Chu, Rev. Sci. Instrum., 60, 845 (1989)]. In both Brown, and Dhadwal and Chu, the end face of the monomode fiber is placed at the focal plane, also known as the Fourier plane, of the lens. This results in a well-collimated beam and hence well-collimated scattering volume. According to Brown, this configuration yields maximum spatial coherence. See Brown, U.S. Pat. No. 4,975,237 (using monomode optical fiber) Dhadwal and Chu, on the other hand, emphasize higher scattered intensity at the expense of spatial coherence to provide accurate results. See Chu and Dhadwal, U.S. Pat. No. 4,983,040; and Dhadwal, U.S. Pat. No. 5,155,549. Further, they teach that in some cases, using multimode fiber instead of monomode fiber produces better results.
Wiese and Horn [J. Chem. Phys., 94, 6429 (1991)] teach another backscattering DLS system using a lensless backscattering probe having a monomode optical fiber and a directional coupler. This configuration is reported to work well in studying high concentration samples in which the scattering volume is placed at the tip of the fiber. Dhadwal et al. [Rev. Sci. Instrum., 62, 2963 (1991)]I disclose yet another system using two monomode fibers in a lensless. In these lensless designs, the laser beam launched from the monomode fiber diverges with the numerical aperture of the fiber. This produces an overly large, relatively uncollimated illumination volume. Analogously, the lensless collecting optical arrangement produces an overly large and relatively uncollimated detection volume. The results in an overly large scattering volume.
There are several patents on DLS systems using monomode optical fibers. U.S. Pat. No. 4,975,237 to Brown teaches a DLS system in which an end face of a monomode optical fiber is positioned at the focal plane of a lens. This arrangement uses the Fourier transforming property of the lens. The same Fourier transforming principle is also taught in U.S. Pat. No. 4,983,040 to Chu and Dhadwal in a multiangle spectrometer and in U.S. Pat. No. 5,155,549 to Dhadwal in a backscattering probe. In both patents, the scattering volume is created by crossing two beams or volumes that are collimated.
U.S. Pat. No. 5,155,549 to Dhadwal and U.S. Pat. No. 5,284,149 to Dhadwal and Ansari disclose backscattering DLS probes having monomode optical fibers and no lenses. The fibers create diverging illumination and detection volumes. By angling the end portions of the fibers inward, the resulting scattering volume is somewhat minimized.
In the field of eye diagnostics (e.g., cataractogenesis, uvetis, and diabetic retinopathy) devices using DLS have elaborate instrumentation and bulk optics. Such DLS devices have optical alignment problems, statistical errors in data analysis, high patient radiation exposure and multiple scattering problems associated with mild and severe cataracts and the polydisperse nature of the cataract itself.
Lensless backscatter fiber-optic probes have been used to study cataractogenesis in eye lenses. However, for reasons stated above, the probe must be brought into close and uncomfortable proximity to the corneal surface of the eye. Moreover, it is very difficult to accurately pinpoint a desired location in the eye. This is because both the effective depth of the scattering volume and the maximum thickness of an adult human lens are about 0.5 mm. Further, the expanding incident laser light tends to disadvantageously illuminate the entire lens. Still further, when the probe is moved beyond about 2.5 mm from the eye/air border, backreflection starts to cause a significant amount of distortion in the collected scattered light. This backreflection increases the photon count rate in the detector and reduces the spatial coherence values. Because of these limitations, in-vivo measurements can only be performed in the front part (anterior cortex) of the eye lens. The nucleus of the eye lens, the posterior cortex, and the vitreous body cannot be probed or accessed reliably by the lens-less probe.
Therefore, it has been deemed desirable to develop a DLS system that has high sensitivity and high spatial coherence. Further, it is has been deemed desirable to develop a DLS system that is accurate, compact, easy to manufacture and easy to use.
The present invention contemplates a new and improved laser light scattering apparatus and method that overcomes all of the above noted problems and others and provides integrated transmitting and receiving optical arrangements for accurate characterization of particles size and related information and is simple and economical to manufacture and use.